A survey of indicator parameters to monitor regrowth in unchlorinated drinking water

The objective of our study was to explore microbiological parameters that are suitable as indicators for regrowth in distribution systems that receive unchlorinated drinking water in the Netherlands. Treated water and distributed water at two locations in the distribution system of 28 treatment plants were analyzed for a range of biological parameters. The results demonstrated that Aeromonas, HPCs, and Mycobacterium were significantly (more often) higher and ATP was significantly (more often) lower in the distributed water than in the treated water, whereas cell numbers remained stable during distribution. Average fungal 18S rRNA gene copies did not differ significantly between the treated and the distributed water, but fungal numbers were significantly more often higher in the distributed water than in the corresponding treated water. These results demonstrate that the distribution system has a clear effect on HPCs, Aeromonas, Mycobacterium, ATP and, to a lesser extent, on fungi in drinking water, but not on cell numbers. Furthermore, we noticed that ATP in the treated water is a better predictor for the regrowth level in the distribution system than total, membrane-intact or HNA cell numbers. In contrast to previous studies, we conclude that ATP and cell numbers seem to be poor indicators for regrowth in the drinking water environment, whereas HPCs, Aeromonas, Mycobacterium and, to a lesser extent, fungi appear to be more reliable indicators to monitor regrowth.